1. Field of the Invention
The present invention relates to an imaging apparatus capable of operating either simultaneously or alternately as a bright-field optical arrangement and/or a dark-field optical arrangement, and to an optical probe having the imaging apparatus included therein.
2. Description of the Prior Art
Contaminants in liquid feedstock streams have long been a major problem within the chemical manufacturing industry. Such contaminants may be in the form of either opaque objects, such as solid particulates, or "phase objects", such as gels and transparent inclusions. Phase objects are a particular problem in polymer manufacturing processes. Phase objects do not typically scatter or absorb light and are thus difficult to see in a polymer stream. However, a phase object usually has an index of refraction that is slightly different from the primary polymer stream due to a difference in molecular weight which causes light which passes through the phase object to refract or change direction.
A prior art commercial system, such as that manufactured by Flow Vision, Inc., of Morgantown, Pa., uses bright-field optics to effectively detect opaque objects in a fluid stream. This system is relatively insensitive to phase objects.
The optical principles underlying a bright-field optical arrangement may be understood from FIG. 1. In the bright-field optical system in FIG. 1, viewing from left to right along an optical axis A are a light source S, which can be a light guide or a lamp, a plate having an aperture S.sub.A, a collimating lens L1, and the material or medium M under test. At the right are the detection optics, comprising a pair of imaging lenses L2 and L3, and an output image plane P. A photodetector, such as a charge coupled device (CCD) array camera, can be positioned at the output image plane. Alternately, a coherent fiber optic bundle faceplate can be positioned at the output image plane to convey light to a distant photodetector.
Light passing through the material or medium M under test is imaged at the output image plane P and thus permitted to impinge on the photodetector. Opaque objects in the material M block light from passing therethrough, and thus cause corresponding dark regions at the image plane, making such objects readily detectable. Hence, a reduction of photons at the photodetector indicates the presence of something in the field of view that is absorbing or blocking the light passing through the medium.
Phase objects can be optically detected using dark-field detection techniques, such as Schlieren methods, which offer significant advantages over bright-field imaging. A description of Schlieren methods may be found in Jenkins, F. A. and White, H. E., Fundamentals of Optics, 4th Edition, McGraw-Hill, New York, 1976, p. 602 and in Vasil'ev, L. A., Schlieren Methods, Halstead Press, New York, 1971. U.S. Pat. No. 4,121,247 (Henry) discloses a Schlieren dark-field detection system. Dark-field detection techniques are typically unable to detect opaque objects.
The optical principles underlying a dark-field optical arrangement may be understood from FIG. 2. In a homogeneous transparent medium (i. e., one that does not vary in its index of refraction) light travels in a straight line. A transparent object or region in the medium M having an index of refraction slightly different from the medium will cause light passing though it to change direction slightly. Phase objects, although usually transparent, frequently have a slightly different index of refraction from the surrounding polymer stream. When a light ray passes through a phase object, the ray direction vector is refracted according to Snell's law. Even a very small change in the index of refraction can cause a surprisingly large change in the direction of an incident light ray. This change in the direction of light propagation provides a mechanism for detecting phase objects.
FIG. 2 illustrates a dark-field telecentric optical system wherein light rays pass through a material or medium under test parallel to an optical axis A. Viewing from left to right along the optical axis A are a light source S, which can be a light guide or a lamp, a plate having an aperture S.sub.A, a collimating lens L1, and the material or medium under test M. At the right are the detection optics, comprising a lens L2, a beam stop B, which blocks the undisturbed light rays, an imaging lens L3, and an output image plane P. A photodetector, as a charge coupled device (CCD) array camera, can be positioned at the output image plane. Alternately, a coherent fiber optic bundle faceplate can be positioned at the output image plane to convey light to a distant photodetector.
The undisturbed rays are blocked by the beam stop B, and only refracted rays R are imaged in the output image plane P and thus permitted to impinge on the photodetector. Hence, the presence of photons at the photodetector indicates the presence of something in the field of view that is changing the direction of the light in the medium. Phase objects and related artifacts appear as bright objects in a dark background at the image plane, making them readily detectable. However, because diffraction can also cause this same effect, such a system is sensitive to very small solid objects that diffract light, as well as refracting artifacts.
Although FIG. 2 illustrates a dark-field telecentric optical system it should be understood that a dark-field focused beam optical system, wherein the light rays converge as they pass through the object plane, may be used. Again, when the optical path through the test material contains no artifacts that disturb the light transmission all light rays are blocked by the stop B and the output image is dark.
In view of the foregoing it is believed advantageous to provide an optical probe having an imaging apparatus capable of operating either simultaneously or alternately as a bright-field optical arrangement and/or a dark-field optical arrangement. It is also believed advantageous to provide an optical probe able to operate within the hostile environment of a complex chemical process.